RFID systems are used in a wide variety of applications, such as for automatic identification in the supply chain, inventory management in a warehouse and monitoring and tracking work in progress in a manufacturing environment. A typical RFID system has RFID tags, each having a unique identifier stored in the tag's memory, and an RFID reader. The RF reader emits an RF signal that is received by an RFID tag within the reader's range. The RFID tag, in turn, is powered by the RF signal which is emitted by the reader; the tag rapidly changes its reflectivity such that the signal reflected back to the reader by the tag is amplitude-modulated, and the unique identifier stored in the tag's memory can be encoded into the reflected signal and therefore be received by the reader. The RFID tag is attached to an object. By reading the unique identifier stored in the RFID tag's memory, the object can be identified, and managed or tracked.
The RFID reader includes an RF architecture for preparing a signal, amplifying the signal and radiating the signal, as shown in FIG. 1. FIG. 1 illustrates an RF architecture in a conventional Ultra High Frequency (UHF) RFID reader. Referring to FIG. 1, a continuous wave (CW) signal generated by a local oscillator 4 is split into two signal paths (i.e., transmit path and receive path) by a power splitter 6. The transmit path includes a pre-amplifier 8, a modulation attenuator 10 controlled by a control signal 28, a power amplifier 1 2, and a filter 14. The receive path includes a filter 20, an amplifier 22, and a mixer 24. A signal from the transmit path is provided to the antenna 18 via a circulator 16, and a signal received from the antenna 18 is provided to the receive path via the circulator 16.
The transmit signal is AM modulated (amplitude modification) using the attenuator control signal 28 to set the modulation attenuator 10 to one of two states so that information can be encoded onto the CW, when the RFID reader is required to transmit information or control signals to the tag. The modulated signal is amplified by the power amplifier 12. After the transmit signal has been filtered to remove unwanted signal components by the filter 14, the signal is fed into the circulator 16, and is radiated by the antenna 18. If an RFID tag is within the read range, said tag will respond to the RFID reader's modulated signal radiated from the antenna 18, and at the appropriate time modulate its response onto the unmodulated carrier (CW) signal transmitted by the RFID reader.
The circulator 16 directs a signal received from the antenna 18 to the filter 20 which removes unwanted signal components. The resulting signal is amplified by the amplifier 22, before being mixed at the mixer 24 with the CW generated by the local oscillator 4, to remove the carrier leaving only the information modulated onto the CW by the tag. The output 26 of the mixer 24 is sent to be demodulated.
The overall amplitude of the transmitted signal, and therefore the read range of the RFID reader is controlled by the power amplifier control 30. RF power amplifiers generally only provide linear amplitude control over a portion of their dynamic range (the linear region), and the resolution of control possible is limited at lower levels of amplification. Therefore the lowest output power of the radiated signal is usually at the lower end of the amplifiers linear region.
FIG. 2 illustrates a typical curve for power output vs power amplifier control signal voltage for a UHF RF power amplifier (Reference: M/ACOM MAAPSS0095 datasheet). The output power is largely linear in the region when the control voltage is between 1.7V and 2.2 V, and this is the linear region of the power amplifier. The curve becomes steep below 1.5V and precise control is therefore not possible, which limits the minimum output power of the RFID reader to the lowest point of the linear region, if accurate control of the output power is to be retained.
As described above, the read range of a UHF RFID reader is usually controlled by reducing the amount of amplification provided by a power amplifier (e.g., using control voltage 30 to control amount of amplification provided by amplifier 12 of FIG. 1) in the transmit path of the RFID reader. The minimum read range is usually defined by the point at which the power output of the amplifier can no longer be linearly controlled, leading to a minimum read range which is greater than that required to scan a single tag when it is surrounded by other tags.
Accordingly, when accurately reading a tag of interest among a plurality of tags, it is necessary to use a costly and elaborate power amplifier which can retain a linear response over a wide dynamic range or to use an inexpensive power amplifier, and perform time consuming calibration of a non-linear region of the power amplifier.